1. Field of the Invention
The present invention relates generally to a method and apparatus for measuring the velocity of a land vehicle, and in particular, to a method and apparatus for measuring the velocity of a land vehicle using an accelerometer and RGI (Route Guidance Information) data.
2. Description of the Related Art
In general, vehicles, such as aircraft, ship, land vehicle, etc., are equipped with navigation systems for positioning the vehicles and providing destination routing. To position a vehicle and provide a route to a destination, a navigation system must decide the position of the vehicle accurately.
Therefore, the navigation system is usually provided with a measuring device for determining its own position. The measuring device can determine the position using an external aid, or using an internal sensor. An example of the case of the external aid is a GPS (Global Positioning System), and an example of the case of the internal sensor is a DR (Dead Reckoning) system using an inertial sensor.
The DR system with an inertial sensor or an Inertial Navigation System (INS) is a navigation system first developed by the Massachusetts Institute of Technology (MIT) in the U.S. in the early 1950s and deployed for practical use in the 1960s. The INS calculates the velocity and position of a vehicle using a gyroscope for sensing rotation and an accelerometer for sensing linear movement.
The basic operational principle of the INS may be summarized as follows. The INS first calculates the vehicle direction (angle) of a vehicle by integrating angular velocities output from the gyroscope, compensates the output of the accelerator for the gravity acceleration, and integrates the compensated accelerator outputs. Thus, the INS autonomously calculates the current velocity and position of the vehicle. While the INS advantageously provides accurate and continuous navigation data for a short term, errors are accumulated over time due to the integration. Therefore, a very expensive accurate gyroscope and accelerator are required to implement the INS. To ensure higher accuracy and long-term stability, the INS is usually used in conjunction with a non-inertial auxiliary sensor such as a magnetic compass and the GPS rather than independently.
As described above, the velocity of a vehicle is calculated by combining velocity information from an accelerator with the vehicle direction information from a gyroscope. Accurate calculation of the velocity of the vehicle in a three-dimensional space requires three one-axis gyroscopes arranged perpendicular to one another and three one-axis accelerators, also arranged perpendicular to one another. Yet, the vehicle velocity can be obtained with the use of fewer sensors according to the type of the vehicle. For example, in the case of a land vehicle, since the rolling of a shaft connecting the front to the rear of the land vehicle and its linear movement in a perpendicular direction to a road surface are negligible, as many sensors as used to sense the rolling and linear movement can be saved.
To calculate a velocity vector of a vehicle traveling on a road, the DR system with an inertial sensor must measure the direction of the vehicle and the velocity of the vehicle in a direction of motion. A gyroscope on an axis perpendicular to the plane of a vehicle shaft is required to measure the direction of the vehicle. Also, to measure the velocity of the vehicle along the direction of motion, the inclination angle of the road must be measured in addition to the use of an accelerator installed in the direction of the vehicle shaft.
The reason for measuring the inclination angle of the road in calculating the velocity vector of the vehicle in the DR system is to calculate the gravity acceleration involved in the output of the accelerator from the road inclination angle. That is, because the gravity acceleration is applied in a direction perpendicular to the surface of the elliptical earth (the surface of a sphere perpendicular to the gravity acceleration), if the axis of direction of the accelerator, fixed in a predetermined direction on the vehicle, is changed due to a change in the road inclination angle, a gravity acceleration component in the output of the accelerator is also changed. Therefore, the pure motion acceleration of the vehicle is achieved only when the gravity acceleration, varying with the road inclination angle, is removed from the output of the accelerator. However, because the motion acceleration in the direction of motion and the gravity acceleration component cannot be distinguished from each other without knowledge of the road inclination angle, an error of the gravity acceleration component is involved in measuring the velocity of the mobile terminal.
FIG. 1 illustrates compensation of a vehicle accelerator output for the gravity force. Referring to FIG. 1, a road inclination angle is defined as an angle from a plane 10 perpendicular to the direction of the gravity acceleration (±g). In FIG. 1, the road inclination angle is an angle θ between the gravity acceleration direction ±g plane 10 and a plane 20 extended in the direction of motion of a vehicle 30.
As the vehicle 30 travels on the plane 20 inclined at the angle θ with respect to a plane 10 perpendicular to the gravity acceleration direction ±g, a measurement {right arrow over (a)} from an accelerator provided in the vehicle 30 is determined by Equation (1){right arrow over (a)}={right arrow over (r)}{right arrow over (a)}+{right arrow over (g)}  (1)
The measurement {right arrow over (a)} includes an actual acceleration component {right arrow over (ra)} and a gravity acceleration component {right arrow over (g)}. This gravity acceleration component, {right arrow over (g)}, which is measured together with a variation in an actual velocity, is an error factor for velocity measuring.
Hence, the vehicle DR system must subtract the gravity acceleration {right arrow over (g)} from the acceleration measurement {right arrow over (a)} to measure an accurate velocity of the vehicle and the road inclination angle is required to measure the gravity acceleration {right arrow over (g)}. To this end, the DR system must be additionally equipped with a gyroscope or a clinometer.
Traditionally, a gyroscope is responsible for measuring a road inclination angle. Therefore, the vehicle-DR system uses two or more gyroscopes: one for deciding the direction of a vehicle and another for calculating the road inclination angle. Since the gyroscopes are basically sensors for measuring variations, the vehicle DR system, which measures the road inclination angle using a gyroscope, integrates the outputs of the gyroscope to obtain the road inclination angle. Therefore, an error component of the gyroscope is integrated in the integration process, thereby accumulating errors over time in estimating the road inclination angle.
To calculate the road inclination angle, thus, the gyroscope is used not alone, but in conjunction with an auxiliary sensor without errors being accumulated. Although a sensor such as a clinometer can be adopted to calculate the accurate road inclination angle, an existing accelerator is generally used as the auxiliary sensor to minimize the number of sensors used.
FIG. 2 illustrates an operation for measuring a gravity component from the output of an accelerator in a conventional system. Referring to FIG. 2, the conventional system measures a relatively low-frequency gravity acceleration component (a) by passing the output of an accelerator including the gravity acceleration component (a) and an actual acceleration component (b) through a low pass filter (LPF) 40, and calculates a road inclination angle using the gravity acceleration component (a).
Despite the advantage that no errors are accumulated in the road inclination angle information, however, this method is sensitive to the performance of the accelerator and insensitive to the change of the inclination angle. Moreover, the use of a low cut-off frequency to separate the gravity component leads to a time delay.
As described above, the conventional vehicle DR system uses an additional gyroscope for estimating a road inclination angle because the road inclination angle cannot be calculated only using an accelerator. However, the addition of an expensive gyroscope to the DR system makes it impossible to realize a low-price DR velocity measuring device.